Known as synthetic resins excellent in transparency are polycarbonates, polymethyl methacrylate, polyethylene terephthalate, etc. For instance, polycarbonates are resins which are excellent in transparency as well as in heat resistance, heat ageing characteristics and impact strength. However, polycarbonates involve such a problem that they are poor in chemical resistance as they are easily attacked by strong alkali. Polymethyl methacrylate has such problems that it is readily attacked by ethyl acetate, acetone, toluene or the like, swells in ether and, moreover, it is low in heat resistance. Though polyethylene terephthalate is excellent in heat resistance and mechanical properties, it involves such problems that it is weak in resistance to strong acid or alkali and is subject to hydrolysis.
On one hand, polyolefins which are widely used as general-purpose resins are excellent in chemical resistance and solvent resistance as well as in mechanical properties. However, many of polyolefins are poor in heat resistance and are poor in transparency because they are crystalline resins. In general, to improve polyolefins in transparency, there is employed a procedure in which nucleating agents are incorporated into polyolefins to render their crystal structure microcrystalline, or a procedure in which polyolefins are quenched to stop the crystal growth thereof. However, it is hard to say that the alleged effects obtained by these procedures are sufficient. The procedure of incorporating into polyolefins a third component such as nucleating agents rather involves the risk of marring various excellent properties inherent in polyolefins, and the quenching procedure requires a large-scale apparatus therefor and, in addition thereto, involves the risk of lowering heat resistance and rigidity as the crystallinity index of polyolefins decreases. That is, in either case, it is not possible to perfectly control the crystal growth of polyolefins, and there remain such problems that the molding shrinkage factor of the thus treated polyolefins is high, and further that the post-shrinkage of molded articles obtained after molding said polyolefins is also high.
Under such circumstances, a copolymer of ethylene and 2,3-dihydroxydicyclopentadiene has been disclosed as an example of copolymers of ethylene and bulky comonomers, for example, in U.S. Pat. No. 2,883,372. However, this copolymer is poor in heat resistance as it has a glass transition temperature in the vicinity of 100.degree. C., though said polymer is well balanced between rigidity and transparency. Similar drawback is also observed in copolymers of ethyllene and 5-ethylidene-2-norbornene.
Japanese Patent Publn. No. 14910/1970 proposes a homopolymer of 1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene. The proposed polymer, however, is poor in heat resistance and heat ageing characteristics. Japanese Patent L-O-P Publn. No. 127728/1983 further proposes a homopolymer of 1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene or copolymers of said cycloolefin and norbornene type comonomers, which are apparently those obtained by ring opening polymerization (ring opening polymers) in light of the disclosure in said publication. These ring opening polymers which have unsaturated bonds in the polymer main chain, however, have such a drawback that they are poor in heat resistance and heat ageing characteristics.
In the course of these researches, we found that cycloolefin type random copolymers of ethylene and specific bulky cycloolefins are synthetic resins which are well balanced between heat resistance, heat ageing characteristics, chemical resistance, solvent resistance, dielectric characteristics and mechanical properties, and that said cycloolefin type random copolymers exhibit excellent performances in the field of optical materials such as optical memory disc and optical fiber. On the basis of the above findings, we have already made various technical proposals as disclosed in Japanese Patent L-O-P Publn. No. 168708/1984, Japanese Patent Appln. Nos. 220550/1974, 236828/1984, 236829/1984 and 242336/1984. In spite of their being olefin type polymers, the cycloolefin type random copolymers as proposed are excellent in adhesion to various materials. However, when these cycloolefin type random copolymers were used for molding information recording base boards under severe conditions, they were sometimes found insufficient in adhesion to information recording films.
As regards information recording base boards (hereinafter sometimes abbgreviated to "optical discs"), polymethyl methacrylates, polycarbonates, polystyrenes, rigid polyvinyl chlorides, epoxy resins, etc., have heretofore been used as resin materials to constitute optical discs. However, optical discs molded from these resin materials individually have a fairly large number of drawbacks. For instance, polymethyl methacrylates are poor in heat resistance and high in water absorption as high as 0.4%. Therefore, molded articles obtained from polymethyl methacrylates are subject to dimentional change owing to their moisture absorption and liable to warp. Polycarbonates are high in modulas photoelasticity as well as in double refraction, even though they are excellent in heat resistance. Moreover, polycarbonates are low in surface hardness and liable to marring, and the problem of moisture resistance still remains unsolved, though the water absorption of polycarbonates is as low as 0.15% in comparison with polymethyl methacrylates. Polycarbonates are practically unusable as optical discs, because they are poor in heat resistance, impact resistance and surface hardness, not to speak of their high double refraction after molding. Polyvinyl chlorides are difficult to use as optical discs, because they are very poor in heat resistance and, moreover, poor in processability and service durability. Epoxy resins involve difficulty in point of mass production as they are poor in moldability, though they are excellent in heat resistance. Moreover, epoxy resins are high in modulus of photoelasticity and involve the problem of double refraction due to residual stress produced at the time when they are molded. Glass is fragile and easily broken and, moreover, it is poor in easy handling or productivity as it is relatively heavy in weight, though it is excellent in heat resistance, moisture resistance and surface hardness.